public class uploadattachedfiles 
{
    Public Attachment myfile;

Public Attachment getmyfile()
{
     myfile = new Attachment();
     return myfile;
}
   
Public Pagereference Savedoc()
{
    String accid = System.currentPagereference().getParameters().get('id');
    Attachment a = new Attachment();
    a.parentId='00128000005yJi7';
    a.name=myfile.name;
    a.body = myfile.body;
    insert a;
    return NULL;
}   
}

-----------------------------------------------------

@isTest
public class testuploadattachment {
    
static testmethod void testattached()
{
   // account ac=new account();
    //ac.name='sanfransico';
    //ac.phone='899899898';
     //insert ac;
    //opportunity op=new opportunity();
    //op.description='hello world';
    //op.Name='sundhar';
    //op.StageName='discriminating';
    //op.CloseDate=system.today();
     //insert op;
    //Attachment acd = new Attachment();
    //acd.name='atach';
   //Blob bodyBlob=Blob.valueOf('Unit Test Attachment Body');
    //acd.body=bodyblob;
    //acd.parentId=ac.id;
    //insert acd;
    
   Test.startTest();
    // string businessName = ApexPages.currentPage().getParameters().put('businessValue', lstLineItems[0].name);
   //PageReference ref = page.uploadattachemntss; 
   //Test.setCurrentPage(ref);
    uploadattachedfiles upd = new uploadattachedfiles ();
   upd.getmyfile();
    upd.Savedoc();
   list<Attachment> ad=new list<Attachment>();
   ad=[select id,name,body from Attachment];    
   // upd.myfile=ad;
    test.stopTest();
   }
}

----------------------
error
System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [Name, Body]: [Name, Body]